When Rare Is Everywhere: Why Small Patient Populations Drive Big Progress

Rare diseases are often defined by numbers: in the United States, a condition is considered rare if it affects fewer than 200,000 people. Individually, each disease impacts a small population. Collectively, rare diseases are anything but rare.

More than 10,000 distinct rare conditions affect hundreds of millions of people worldwide. Nearly every family is touched in some way, through diagnosis, caregiving, or the broader impact on health systems and communities.

Despite this reach, rare diseases have historically remained on the margins of medical research and drug development. Small patient populations, complex biology, and uncertainty around clinical development have made them difficult to pursue.

That is beginning to change.

Rare diseases demand an unusually deep understanding of human biology. Because symptoms are frequently severe, early-onset, and multifaceted, researchers must untangle core disease mechanisms to make progress. That rigor can generate insights that extend beyond a single condition.

Advances in genetics, neuroscience, and precision medicine are reinforcing what many researchers have long understood: breakthroughs often begin with the smallest patient populations. Insights from rare neurological and genetic disorders have informed therapies for diseases that are more common and reshaped how the industry approaches diagnosis, treatment, and care.

In this way, rare disease research strengthens the very foundation of medicine.

Rett syndrome offers a compelling example.

Rett syndrome is a rare and complex neurodevelopmental disorder that primarily affects girls. It is estimated to affect approximately 6,000 to 9,000 individuals in the United States.

The condition is typically caused by mutations in a single gene, MECP2.

What are the Symptoms of Rett Syndrome?

Rett syndrome results in a wide array of severe symptoms, including:

• Loss of speech

• Loss of motor skills

• Seizures

• Breathing irregularities

• Significant impairment of daily functioning
  

Diagnosis can be challenging. Early signs may resemble other developmental or neurological conditions. It can take two to four years from the first presentation of symptoms to reach a definitive diagnosis.

As the condition progresses, children often lose previously acquired skills and experience, increasing neurological challenges over time.

Studying Rett syndrome requires understanding how the brain develops, including how neurons communicate, how neural circuits mature, and how early disruptions affect lifelong function. Decades of academic and clinical research in Rett syndrome, including work on neurodevelopmental pathways, have contributed to broader understanding of brain development and function relevant to other neurological and developmental conditions.

What is Life Like for Families & Caregivers Living with Rare Diseases?

Rare diseases do not affect individuals alone. They affect families and caregivers too.

Caregivers often become de facto experts, coordinating care across multiple providers while advocating for research, awareness, and support. Rett syndrome is a lifelong condition associated with serious medical vulnerability over time, including seizures, breathing problems, difficulty swallowing, and limited mobility. Individuals often require coordinated care from multiple specialists throughout their lives.

Each diagnosis affects parents, siblings, educators, clinicians, and entire communities.

Progress in research and awareness matters – not only for treatment advancement, but also for connection.

How is Rare Disease Research Shaping the Future of Medical Innovation?

In recent years, more biopharmaceutical companies have recognized the broader value of focusing on rare diseases, particularly in neurology.

At Acadia, we have held a longtime commitment to rare and neurological diseases, enabling us to build capabilities, relationships with clinicians and patient communities, and institutional knowledge well before today’s surge of interest. That early conviction matters. It has empowered us to advance care for underserved neurological and rare disease communities through long-term investment, rigorous science, and close collaboration with patient communities. For organizations willing to invest with discipline and long-term confidence, rare disease represents one of the most resilient and impactful frontiers in biopharma today.

That long-term commitment comes to life in the programs we choose to pursue. Our work in Rett syndrome reflects a commitment to ensuring research efforts remain aligned with the lived experiences of families, as we recognize that meaningful innovation depends as much on listening as it does on discovery.

Importantly, the focus on rare disease does not end with a single condition. The principles applied in Rett syndrome – evidence-based science, community engagement, and sustained investment – are shaping how research organizations approach other rare and neurological diseases as understanding of the brain continues to evolve.

The broader message is clear: rare disease research is not a niche pursuit, and the opportunity has never been more compelling. It is a catalyst for innovation. Progress for small populations often illuminates the path forward for many more.

Supporting the rare disease community begins with awareness and grows through collective action.

Zebras are a symbol of rare diseases and conditions. No two zebras are alike, just as no two rare disease journeys are identical. When zebras come together, they form a “dazzle,” a collective made stronger by every individual stripe.

In recognition of Rare Disease Month and Day, Acadia is assembling a Rare Dazzle mosaic to celebrate the strength, resilience, and individuality of the rare disease and Rett syndrome communities. Each zebra added to the Rare Dazzle mosaic represents a story, a voice, and a shared commitment to standing together.

Community members can participate by downloading a zebra, designing it, and giving it a unique name. A photo of the designed zebra can then be uploaded to the Rare Dazzle Mosaic at RareDazzle.com.

The zebra can continue to travel, on adventures, vacations, or everyday errands, with additional photos added to the mosaic to show how far the Dazzle can go.

As we at Acadia know, when individuals unite in support of rare disease awareness, something truly dazzling happens.

When society supports research, listens to patient communities, and fosters collaboration, it affirms a powerful idea: every patient matters – no matter how small the population.